Ronaldo limped out after 73 minutes of the game with a muscle tear. The Brazilian ruled himself out of Real's Champions League game against Bayern Munich next week and said he feared a lengthy absence.
Valencia took full advantage of a dispiriting 102nd birthday for Real by claiming a 3-0 win at home to Deportivo Coruna with two goals from Vicente and one from Juan Sanchez.
With 11 games left to play, Real lead the standings with 60 points, followed by Valencia on 54 and Deportivo on 50.
Barcelona, in fourth place on 46 points, can move to within a point of Deportivo if they win at home to Real Mallorca on Sunday.
Real took the lead against Racing after 28 minutes when Zinedine Zidane and Ronaldo combined to set up Santiago Solari but Yossi Benayoun equalised four minutes later, sweeping in at the far post when Real failed to clear a lofted free kick.
Carlos Queiroz's side had a couple of decent chances but David Beckham and Zidane were both off colour, after struggling with illness this week, and, with Raul already missing, Ronaldo's injury left them with little hope of claiming a second goal.
"We had clear chances and we managed to keep the ball and control the game but we couldn't take the three points," said Queiroz. "It just wasn't possible."
Real had defender Alvaro Mejia sent off in injury time but Queiroz will be more concerned with the state of Ronaldo's thigh ahead of Wednesday's Champions League date with Bayern.
"I'm completely ruled out for Wednesday," Ronaldo told Radio Marca. "I think it's going to be quite a long period (of recuperation).
"It was a pretty bad strain and it's a concern."
Celta Vigo, who play Arsenal in the Champions League next week, lost 5-1 at home to Espanyol in a defeat that leaves them in 18th place and in real danger of relegation.
Real Sociedad, in the relative security of mid-table, warmed up for their Champions League game against Olympique Lyon next week with a 1-1 draw away to Osasuna.
Valencia were a lot less convincing than the scoreline in their game against Deportivo would suggest.
Ruben Baraja missed an early penalty but Vicente took his chance from the spot in the 21st minute after Mauro Silva had brought down Mista on a run into the box.
Silva was sent off for his troubles but Deportivo had plenty of chances to equalise before injury-time goals from Vicente and Juan Sanchez settled matters.
